Traditional telephone arrangements whereby a number of lines may be accessed from a single station instrument are called key telephone systems. They are located on the customer's premise and are generally connected either to telephone lines originating from central office switching equipment or from a local PBX. In either instance the telephone line consists of a pair of conductors, a tip conductor and a ring conductor. In such systems line circuits such as the line circuits shown in U.S. Pat. No. 3,436,488, issued to R. E. Barbato and D. T. Davis on Apr. 1, 1969, U.S. Pat. No. 3,647,983, issued to A. R. Fitzsimons and R. J. Phelps on Mar. 7, 1972, and U.S. Pat. No. 3,895,192, issued to R. J. Angner and A. Feiner on July 15, 1975, are connected between the station and the telephone line for the purpose of controlling the line to station connection. Such line circuits typically are comprised of logic circuitry and either three or four operating relays, and further include circuitry for delivering power, for ring detection, and for providing a visual indication of line status. One line circuit is ordinarily required for each telephone line serving the system, while each line circuit may serve one or more telephone station sets. Thus, particularly as the number of telephone lines serving a customer's premise increases, the cost as well as the physical size of the line circuit becomes an important factor in determining the cost and compactness of the overall key system installation. Designers and inventors, including those referenced in connection with the patents cited above, over the years have attempted to improve the reliability and reduce the size and the cost of line circuits by employing solid state electronics and minimizing the number of large, costly and failure prone mechanical components required for line circuit operation. This is reflected, for example, in the Fitzsimons and Angner patents which reduced the number of relays employed in conventional line card technology from four (see the Barbato patent) to three.
Notwithstanding the achievements in conventional key system technology over the years, recent regulatory changes and marketplace developments have imposed new demands on key system technology heretofore not considered critically important in the design of prior art line circuits. The Federal Communication Commission has determined that the direct connection of terminal equipment should not introduce any harmful or potentially degrading effects onto the public telephone network facilities represented, for example, by the basic telephone line from the central office to the customer's premise. Among the parameters required by the Federal Communication Commission (F.C.C.) to be met and maintained by directly connected telephone equipment, including key telephone systems, are minimum limits on longitudinal balance. According to Section 68.310 of the F.C.C.'s Rules and Regulations (47 CFR .sctn.68.310) prescribed minimum balanced conditions, measured by the coefficient of metalic-to-longitudinal balance must be satisfied in all normal operating states of the equipment. In key systems the line circuit is one of the most significant system components effecting the balance parameter, and conventional line circuit designs have been shown recently to have difficulty meeting the F.C.C.'s prescribed minimum standards.
The metallic-to-longitudinal balance coefficient relates longitudinal balance at the terminal equipment to induced energy in other loops or lines, and therefore has been determined to be the appropriate measure of potential for harmful cross talk or noise interference with telephone service. The longitudinal balance limitations recently made applicable to terminal equipment are predicated upon the parallel connection of the equipment to complete a "loop" comprised of the tip and ring conductors of the telephone line bridged by the equipment at the termination of the line. Where a series impedance (e.g., a relay coil) is interposed on one conductor path of the telephone line (either tip or ring), that impedance upsets the otherwise longitudinally balanced state of the equipment (e.g., a station set) with which it is in series, as perceived by the telephone network. Balance is, further, a function of both series (i.e., resistive) unbalance and shunt (capacitive) unbalance to ground. More specifically, series unbalance may be defined as the series impedance difference between tip and ring, and can be a significant cause of longitudinal imbalance in circuits characterized by low longitudinal impedance. Shunt capacitive unbalance is the most general cause of line-to-terminal equipment imbalance and is defined as the difference between tip-to-ground capacitance and ring-to-ground capacitance. In key telephone systems the line circuit is a potential source for both kinds of imbalance.